Laser plotters are used extensively in producing transparencies where a large amount of sequentially printed data is involved, such as the production of seismographs.
FIG. 1 illustrates a laser graphics plotter of the type commonly in use in the art. Collimated light source 2 produces a light beam which is focused by lens 3 onto the surface of film 4 as it passes over photodrum 5. Film 4 is unwound from film roll 6 and is wound on take up tube 7 as film 4 is exposed. Take up tube 7 is illustrated as being contained in a light blocking container 8. Similarly, film roll 6 is contained in a housing, usually cylindrical (not shown) which has a diameter closely approximating that of the film magazine with film. Photo drum 5 and take up tube 7 are driven respectively by a stepping motor 9, or the like and torque motor 10.
In a laser graphics plotter, a collimated light source, such as laser, generates a beam of light that ultimately will produce the spots on light sensitive film positioned for exposure in an essentially flat field image plane, the surface of the photodrum. The collimated light beam is applied to a light intensity modulator, the response to the plot data to modulate the intensity of the collimated light beam. An optical assembly is provided between the collimated light source and the light intensity modulator to focus and reduce the beam diameter into the modulator to permit high modulation rates through the light intensity modulator. The modulated beam which exits the modulator passes through a lens which recollimates and focuses the beam to produce the desired spot size on the photodrum.
In some systems, the modulated beam is applied to a rotating multi-facet mirror which scans the modulated light beam across the image plane of the film. Positioned between the rotating mirror and the image plane is a flat field scan lens which provides compensation for the non-linear velocity of the modulated light beam across the image plane and also maintains a constant spot size for each spot plotted.
Wide format laser plotters which plot, in a raster format of consecutive scan lines, selectively position variable intensity spots or dots on light sensitive film positioned for exposure on the photodrum are commonly used in seismograph production. This wide format is traditionally forty two inches wide. The laser plotting systems illustrated uses a stepping motor as the primary motivating force for advancing the film across the photodrum. For these plotters, at the completion of a scan line, the film and film advancing mechanism is moved by pulsing the step motor. When the film is abandoned, the holder on which a film magazine is placed provides a predetermined drag. This drag prevents the film magazine from continuing to rotate when the step motor stops, thus keeping the film tightly wound on the film roll. At the completion of the required number of incremental steps to advance the film, the specified line-to-line separation, the film handling assembly is commanded to stop. To produce a given spot size at the image plane, the modulated laser beam is focused on the image plane to provide the desired spot size at the center of the scan angle interval. The beam diameter at that point is equal to the desired spot size.
Normally, a laser plotter is configured to operate using forty two inch film in the production of seismographs. Although forty two inch width seismographs are commonly in use, narrow widths such as thirty inches have become popular. However, in printing thirty inch seismographs, twelve inches of film are wasted since laser plotters are design to operate only with forty two inch film. The present invention provides a method and apparatus for permitting use of film thirty inches wide or any width less than forty two inches, using conventional laser plotters. As a result, through the use of the present invention, the film may be cut to the desired width of the seismograph which results in less waste and greater efficiency.